1. Field of the Invention
This invention relates generally to imaging of liquids, liquid films, liquid droplets, weakly bound particles on surfaces, and surfaces of thick dielectric samples using scanning force microscopy (SFM).
2. Description of Related Art
Atomic force and scanning force microscopes have been used to image solid surfaces with regions of electrostatic charge or dielectric material using metal or metal-coated cantilever probe tips. However no technique has been successfully used, including scanning force microscopy and atomic force microscopy, to image the nanometer-scale structure of liquid films, surfaces, and adsorbates. It is important to understand the growth of thin films of water because water films alter the adhesion and lubricating properties of surfaces and the reactivity of solids with ambient gas molecules. Additionally, water is a common solvent and cleaning agent. A method to study the growth of thin films of water would be important for physics, chemistry, and biology. In chemistry, the contact angle of water is used to measure the chemical activity of the surface. In biological processes, water films are critical for ion transport. In the semiconductor industry, water is used to wash off surface residues including and similar to reactants and acids. It is very important to know of the presence of residual liquid residues on the surface. Several studies have been recently devoted to the layering and orientation of water molecules on surfaces (Q. Du, E. Freysz, Y. R. Shen, Phys. Rev. Lett. 72, 238, 1994; J. D. Porter and A. S. Zinn, J. Phys. Chem. 97, 1190, 1993; J. N. Israelachvili, Chem. Scr. 25, 7, 1985; AC. Chem. Res. 20, 415, 1987; J. Glosli and M. Philpott, Proceedings of the Symposium on Microscopic Models of Electrode-Electrolyte Interface, Electrochem. Soc. 93-5, 90, 1993). Ice-like structures have been predicted for the first layers of water molecules, but no experimental evidence is available to validate the predictions. A method and apparatus to image water surfaces with nanometer resolution would make it possible to study many basic aspects of wetting, including condensation, evaporation and chemical reactions. With such a tool, the validity of growth models and the structure of the first layers could be investigated at the molecular level. If this type of tool were available it would also enable people to map the location of liquid droplets in the nanometer range.
Modern scanning force microscopes (SFM) like the scanning tunneling microscope (STM) and the atomic force microscope (AFM) can be used to obtain atomic-scale resolution of solid surfaces. SFM uses a probe tip attached to a spring, normally in the form of a lever or cantilever. Any of a number deflection sensor systems is used to measure displacement of the spring from its rest position under the influence of external forces, typically coming from the sample to be imaged.
SFM cantilever probe tips are available in a range of force constants (k), typically in the range between about 0.01 and 100 Newtons per meter (N/m). The resonant frequency (f) of the probe tip can typically range from 1 to 200 kHz. Probe tips are generally available with radii (R) of about 5 to about 0.01 micrometers (.mu.m). Many materials have been used for probe tips, including tungsten, platinum, other metal alloys, and semiconductors such as silicon or silicon nitride. Diamond has also been used as a SFM probe material. Conductive coatings have been applied to otherwise non-conducting probe tips.
Sometimes the SFM cantilever probe is used as a null displacement sensor, where a feed back loop in the SFM apparatus applies a compensating force to balance external forces from the sample. Operated in this mode, the SFM cantilever probe is maintained in a position as close to its rest position as possible. As the probe is rastered across the sample, the magnitude of the force required to prevent the probe from deviating from its rest position is measured and plotted as a function of probe location over the sample. The resulting two-dimensional plot is the image obtained under constant distance constraints.
More frequently, however, the SFM cantilever probe is allowed to deflect in response to external sample forces and, as the probe scans the sample, the magnitude of the deflection is measured and plotted as a function of location over the sample. This two-dimensional plot is the image. There are many types of sensor systems that can be used to measure the displacement of the cantilever. One of the most popular is the optical lever, using a segmented or differential photo diode (G. Meyer and N. M. Amer, App. Phys. Lett. V 53, p 1045, 1988; G. Meyer and N. M. Amer, App. Phys. Lett. V 56, p 2101, 1990). Other types of displacement sensor systems include an optical interferometer which may be based on optical fiber technology, electron tunneling sensors, capacitance based sensors, and piezoresistive sensors that directly detect lever deflection.
An SFM can be operated in a contact mode or in a non-contact mode. In the contact mode, the probe tip is pushed against the sample surface so the surface and probe tip are in close mechanical contact. In the contact mode, the lateral spatial resolution is approximately the width of the contact, about 1 nm. In the non-contact mode the probe tip is typically 5 nm to 1000 nm from the surface, a great distance compared to atomic bond distances. The probe tip and sample surface interact by long range forces such as van der Waals, electrostatic and magnetostatic forces. If the probe tip and/or sample are submerged in a liquid, they may interact via variations in chemical or electrochemical potentials. In air the lateral resolution for a probe tip of radius R that is separated from the sample surface by a distance D, is about (2 D R).sup.1/2 if D is smaller than R. If D is larger than R, the lateral spatial resolution is about equal to D.
Because liquid surfaces will be disturbed in a contact mode, liquids must be imaged in the non-contact mode. The SFM is operated in the non-contact mode by measuring any of several parameters. The deflection or displacement of a cantilever probe is one common measurement parameter. Another is modulation of an AC signal applied to the probe. Alternatively, the probe tip can be mechanically vibrated, and the effect of sample interaction with the vibration measured. Monitoring the change in mechanical vibration measures force gradients rather than magnitude. For solid samples, it is also possible to make measurements in a hybrid mode, sometimes referred to as a "tapping" mode, wherein parameters measured for contact and non-contact imaging are combined.
Van der Waals forces are always present and can be used for imaging. However, because they are very weak (the force is about 1 nN at 10 .ANG. distance for a 500 .ANG. probe tip radius), close proximity of the probe tip to the surface is required. Such close positioning to a liquid surface results in jump-to-contact instabilities. Electrostatic forces have a much longer range and magnitude than van der Waals forces and can be easily used to perform non-contact imaging of both conducting and insulating materials.
To use SFM to measure the electrostatic forces resulting from a charge distribution in the sample, a voltage is applied to the probe tip and the repulsion or attraction of the sample to the charged probe tip is measured. The charge distribution on the sample can result from polarizing a dielectric material, depositing fixed or mobile point charges in or on a non conducting sample, presence of differentially conducting domains in a sample, or, for conductors--applying a charge to an electrically isolated conductor or connecting the conductor to an external potential. Electrostatic forces on the sample can be detected with insulating probe tips, however the charge state of an insulating probe tip is difficult to control or characterize so most SFM measurements of electrostatic forces are made using conducting probe tips. Electrostatic forces in the sample can be measured using either dc or ac technology. Imaging of electrostatic fields by SFM has also been referred to as "scanning capacitance microscopy" and "Maxwell Stress Microscopy".
Martin, Abraham, and Wickramasinghe describe use of an AFM with a sharp tungsten probe to measure electrostatic forces with a spatial resolution of about 1000 .ANG. (Appl. Phys. Lett. V. 52, p 1103-5, 1988). When a voltage is applied between the probe tip and the conductive surface that is being imaged, the separation-dependent capacitance between the probe tip and sample creates a capacitive force that is dominant for separation distances larger than of a few tens of angstroms. This methodology is useful to image surface dielectric properties or electrostatic forces of conductive solids or very thin insulators (about 1000 .ANG.) on a conductive substrate. The Martin et al. capacitive type measurements verified the conductor and dielectric pattern of microcircuit on a semiconducting wafer. The conducting elements of the microcircuit comprised the counter electrode. The counter electrode location within about 2,000 .ANG. of the probe tip electrode was intrinsic to their measurement system because the conducting elements were an integral part of the sample that was being measured. Liquid samples could not be configured following their sample set-up.
Saurenbach and Terris describe a mode of AFM imaging for detection of static surface charge in a crystal of Gd.sub.2 (MoO.sub.4).sub.3 (Appl. Phys. Lett. V 56, p 1703-5, 1990). For the probe they use an electrochemically etched tungsten wire with the last 50-100 .mu.m of the wire bent at a right angle to form a cantilevered probe tip. The resonant frequency of the probe tip changes as a function of the electrostatic charge on the sample surface. As the probe tip scans the surface, its resonant frequency is measured to determine the location of static charge. The method of Saurenbach and Terris is limited to measurements of ferroelectric materials where domains are oriented by applying an electric field to the backside of the sample.
Free liquid surfaces cannot be studied using the techniques and apparatus described above. For liquids, if the probe tip comes into contact with the surface, strong capillary forces will cause the liquid to wet the probe tip and strongly perturb the liquid. In order to avoid bulging of the liquid surface that leads to wetting and capillary interaction, the probe tip must be kept at least several tens of angstroms from the imaging surface.
Electrostatic force microscopy has been used to study such properties, as capacitance, surface potential and charge or dopant distribution, in metal and semiconductor samples. It has also been used to deposit and image localized charges on thin film insulators. Although the dielectric properties of surfaces can be studied in this way, the activity in this direction has been limited to very thin, never more than about 2,000 .ANG., layers of insulating materials on conducting substrates.
In all of the scanning force microscopes currently in use the probe tip is polarized relative to a proximate ground or a counter electrode located at least within a couple of thousand angstroms of the probe tip. This limits the sample size to conductors and thin insulators.
In all known uses of SFM the probe and counter electrode have been closer than a few thousand angstroms. Current users of SFM place the probe tip within a few thousand angstroms of the sample surface. No-one has successfully imaged liquids using the conventionally configured SFM. It would be desirable to be able to image non-conducting samples with thickness greater than 0.2 .mu.m. It would also be very desirable to image liquids on insulating substrates. It would be extremely desirable to be able to image bulk liquids, or inhomogeneous liquids with disturbing the surface topology. It would be further desirable to be able to image insulating particles on thick insulating samples.